Corpuscular beam microscope apparatus

ABSTRACT

The beam condenser located in an electron or ion microscope between the beam source and the objective lens comprises at least two lenses, namely a first condenser lens which produces a reduced image of the beam source, and a last condenser lens which produces a reduced image of an aperture diaphragm upon the object plane defined by the objective lens. Within such an arrangement, the first condenser lens comprises a plurality of lens pole shoe systems that can be selectively placed into active position of axial alignment with the beam source and the objective lens. The pole shoe systems have respectively different imaging lengths but have the diameter of their lens bore as well as the width and axial position of their lens gap, so adapted to one another as to maintain an invariable axial position of the source image produced by the first condenser lens, regardless of which one of the pole shoe systems is placed into active position at a time. This affords selecting different radiation (illumination) apertures for one and the same illuminated area of the object without the necessity of changing the focal lengths of the other lenses; and it permits adjusting the size of the illuminated area of the object independently of the magnitude of the illumination aperture of the object.

United @tates Patent 3,5b IL78l [72! Inventor Wolfgang Dieter Riecke()THER REFERENCES Berlin German, Siemens Electron Microscope" publishedby Siemens & 12] I App] 656400 Halske Aktiengesellschaft, Karlsruhepages 1 5 and 6, [22] Filed July 27. I967 v 451 Patented Feb. 2, l97l yy Lake [73] Assignee Max-Planck-Gesellschaft Zur Fordcrung AS51510"! V.Lafranchi Der wissenschamn y Atl0rneysCurt M. Avery, Arthur E. Wilfond,Herbert L. Gottingen. Germany Lerner and Daniel J. Tick a corporation ofGermany gi Pnom) 1967 ABSTRACT: The beam condenser located in anelectron or M72885y ion microscope between the beam source and theobjective lens comprises at least two lenses, namely a first condenserlens which produces a reduced image of the beam source, and a lastcondenser lens which produces a reduced image of an aperture diaphragmupon the object plane defined by the objective lens. Within such anarrangement, the first condenser lens comprises a plurality of lens poleshoe systems that can be selectively placed into active position ofaxial alignment with the beam source and the objective lens. The poleshoe systems have respectively different imaging lengths but have the[54] CORPUSCULAR BEAM MICROSCOPE APPARATUS 17 Claims, Drawing Figs.

U.S. Cl 313/84, di f th i [ens bore 35 we as the width and axial pOSl-2:50/495 tion of their lens gap, so adapted to one another as tomaintain [51] Int. Cl ..H0l 29/46, an invariable axial position f theSource image produced by 1 37/16 the first condenser lens, regardless ofwhich one of the pole Field of Search 250/495 Shoe Systems is placedinto active position at a i i f (4); 313/83 84- fords selectingdifferent radiation (illumination) apertures for one and the sameilluminated area of the object without the [56] References cuednecessity of changing the focal lengths of the other lenses; and

. FOREIGN PATENTS it permits adjusting the size of the illuminated areaof the ob 923,616 2/ I955 Germany.. 250/495 ject independently of themagnitude of the illumination aper- 929,747 7/1955 Germany 250/495 tureofthe object.

' CORPUSCULAR t -BEAM SOURCE MAGNETIC CIRCUIT c /STRUCTURE E T A or e-7o LENS POLE SHOE SYSTEM '4 73 7| INTERPOLE LENS sAP ENTRANC E-8O PUPILPLANE 79 gfifig OBJECTIVE LENS mama] FEB 21971 3.560.781

SHEET 1 0F 4 In vantor:

PATENTED FEB 2|97z SHEET 2 0F 4 4 mm om mm mm Nm m 5 mN mm mmwm mmmm mowum ATENTEU rsa 2m: 3560181 mm 3 0F INVENTOR WOLFGANG DIETER RIECKEPATENTEI] FEB 2197: 7 3,560,781

saw u nr 4 Fig.5

CORPUSCULAR BEAM SOURCE MAGNETIC CIRCUIT STRUCTURE CENTRAL BORE LENSPOLE SHOE SYSTEM 1| INTERPOLE LENs GAP/75 TUR A FIRST ISIESPSLE QNRENSERLENS GAP/ L 72\LENS POLE CENTRAL-74 SHOE SYSTEM BORE OTHER 7a CONDENSER/LENS APERTURED DIAPHRAGM/77 OBJECT 79 PLANE ENTRANCE-#80 PUPIL PLANEOBJECTIVE 1N VENTOR.

LENS WOLFGANG DIETER RIECKE 1 CORPUSCULAR BEAM MICROSCOPE APPARATUS Myinvention relates to corpuscular beam apparatus and will hereinafter bedescribed preferably with reference to electron microscopes. although itis likewise applicable to advantage with ion microscopes. electron orion diffraction equipment. or other corpuscular beam apparatus of thetype comprising a beam source. a condenser lens arrangement and anobjective lens whose lens field determines an object plane. Moreparticularly the invention relates to corpuscular beam microscopeapparatus equipped with at least two condenser lenses, namely a firstcondenser lens (condenser I) which produces a reduced first image of thebeam source, a last condenser lens (condenser III) located between thefirst condenser and the objective lens, and which also comprises anaperture diaphragm, preferably in such a position that the lastcondenser produces a reduced image of the diaphragm aperture on theobject (specimen) coinciding with the object plane.

Such a series of at least two condenser lenses, an aperture diaphragmdetermining the size of the object region illuminated, and an objectivelens in whose lens field the specimen or object to be investigated is tobe located, is known in principle from the paper Ein Kondensorsystem fureine starke Objektivlinse" in the publications of the Fifth IntemationalCongress for Electron Microscopy, [962. A similar microscopic apparatusis described in a paper by W. D. Riecke and E. Ruska, A 100 kVTransmission Electron Microscope With Single-Field Condenser Objectivein the publications of the Sixth lntemational Congress for ElectronMicroscopy, [966. In both papers the last condenser III is shownconstituted, not by a separate lens, but by the always existing prefieldof the objective lens. The utilization of this prefield of the objectivelens is also known from German Pat. Nos. 875,555 and 914,167 inconjunction with different lens constructions and difierent arrangementsof the specimen-object. Reference with respect to the same type ofcombined condenser-objective lens may be had to the copending US. Pat.application Ser. No. 656,402, filed Jul. 27, 1967, of W. D. Riecke whichis now US. Pat. No. 3,508,049 granted Apr. 21, 1970.

The present invention thus relates not only to apparatus with a discretelens arrangement for the last condenser [II but also to apparatus inwhich the prefield of the objective lens is used as a last condenser IIIof short focal length, ie for producing an image of reduced scale.

Furthermore, a third condenser (condenser II) of long focal length maybe situated between the condenser I, that immediately follows the beamsource to produce a reduced image thereof, and the last condenser III ofshort focal length consisting of a separate lens or being constituted bythe prefield of the objective lens. The intermediate condenser IItransfers the first reduced beam-source image, produced by the condenserI, substantially to the entrance pupil plane of the objective lens. Ifthe condenser III is constituted by a discrete lens assembly, then thefront focal plane coincides at least approximately with the entrancepupil plane of the objective lens.

Regardless of which particular system of lenses and beam paths is beingemployed, it is an object of my invention to provide in any suchcorpuscular beam apparatus a reliably applicable possibility ofirradiating (illuminating) a minute region of the object (specimen) bythe corpuscular beam and to also adjust a desired radiation(illumination) aperture of the object.

Another purpose of the invention is to permit changing the illuminationaperture for a fixed illuminated region of the object, this fixed regionbeing determined preferably by the choice of the above-mentioneddiaphragm aperture projected onto the object.

Still another purpose of the inventionis to afford changing theillumination aperture of the object by an adjustment in one of thecondenser lenses while requiring no change with respect to the focaliengths of the other lenses.

Still another object of the invention is to permit changing the size ofthe illuminated region on the specimen or object being investigatedindependently of the size of the radiation (illumination) aperture ofthis object, and vice versa.

To achieve these objects. and in accordance with a feature of myinvention, a microscopical apparatus of the above-men tioned types hasits first condenser (condenser I) provided with several selectivelyinsertable pole shoe systems which have respectively different imaginglengths and in which the width and axial position of the lens gap aswell as the diameter of the central pole shoe bore are dimensioned inrelation to each other so that the axial position of the first image ofthe beam source remains invariable when exchanging one of the insertedpole shoe systems for another one of these systems.

It is a notable advantage of such a corpuscular beam apparatus that thefocal lengths of the other above-mentioned lenses need not be changed.This is because the exchange of differently dimensioned pole shoesystems causes only a change in imaging ratio of the condenser I butdoes not involve a change in magnetomotive force (ampere turns) of thiscondenser nor an axial displacement of its imaging plane. Anotherfavorable property of the appanatus according to the invention residesin the fact that the cross section of the corpuscular beam on thespecimen or other object (such as a specimen carrier film), this crosssection constituting the size of the illuminated object area," isadjustable independently of the size of the radiation (illumination)aperture of the object, and vice versa.

This will be presently elucidated with reference to the accompanyingdrawings in which:

' FIG. 1 is an explanatory diagram of the beam path system in acorpuscular beam apparatus, such as an electron microscope, according tothe invention;

FIG. 2 is another explanatory diagram. relating to the properties of oneof the pole shoe systems with which the first condenser lens inapparatus according to the invention is equipped;

FIG. 3 is a diametrical cross section through a condenserlens assemblyin apparatus according to the invention; and

FIG. 4 is a partly sectional plan view of the same assembly.

FTG. 5 is an elevational view partly in section of the corpuscular beamapparatus according to the invention.

The beam system according to FIG. 1 relates to a corpuscular beamapparatus equipped with two condensers of which only the lens planes areindicated at I and II. The two lenses are constituted by discrete lensassemblies. The apparatus further comprises a last condenser III whichis constituted by the prefield of the objective lens whose object planeis indicated at 0. FIG. 1 also shows a cross-sectional curve of themagnetic field of the objective lens. The field represented has abell-shaped configuration, although this is not an indispensablerequirement for apparatus according to the invention. The prefieldportion is identified by crosshatching. The object plane 0 is located atapproximately the middle of the field configuration, correspondingsubstantially to the locality of maximum field strength. It will benoted that the object plane is optically or magnetically determined bythe objective lens in distinction from the specimen plane or thespecimen-bearing plane of the specimen carrier where the specimen orobject being investigated may be located in fact. For investigation of aspecimen, its cross-sectional plane to be microscopically viewed has tobe located in the object plane.

The condenser I having short focal length produces a reduced image rl ofthe beam source 0. More precisely, such a condenser usually forms areduced image of the smallest beam cross section (crossover) in front ofthe emitting cathode surface of the source.

The intermediate condenser II has a longer focal length than thecondenserl or the condenser III and produces at r2 a sharp image of thereduced first image r! of the beam source Q. Located behind thecondenser II, seen in the beam issuing direction, is a limiting aperturediaphragm B. The locality of this diaphragm is within the focal lengthof the image-forming side of the condenser II The prefield condenser lllproduces a sharp image ofthe diaphragm aperture in the object plane 0.

Due to the great image reduction effected by the condenser lll. verysmall illuminated object regions can be obtained with the aid ofdiaphragms B whose aperture diameter from the manufacturing viewpoint.is conveniently large. For example, a'diameter of the illuminated objectregion of 700 A can be readily obtained by employing a diaphragm with anaperture diameter of IO uM., the prefield condenser lll effecting areduction of approximately 1:140.

It is of interest that the illumination aperture of the object I isgiven by the size of the second image r2 of the beam source but isindependent of the aperture size of the diaphragm B. Thediaphragmdetermines only the size of the object area being illuminated,and this area is independent of the size of the image at r2, it beingunderstood that these mutual independencies apply precisely fornegligible lens faults. Since the magnitude of r2 is given by theproduct of the magnifying scales of the two condensers l and II (as arule the condenser II magnifies slightly), the size of the illuminationaperture of the object can be varied by changing the magnification ratioof the condenser I.

In principle, of course, a corresponding change in magnification, usingan electromagnetic lens as condenser I, can also be effected by changingthe magnetomotive force of the condenser I. However, as is known forexample from the paper of Liebmann and Grad Imaging Properties of aSeries of Magnetic Electron Lenses," Proc. Phys. Soc. B 64, 1951), pages956 to 97 l particularly page 963, such change in magnetomotive forcechanges a parameter k which is proportional to the square of the maximalmagnetic field strength in the lens. As a consequence, the position ofthe reduced first image r1 of the source, produced by the condenser I,will change. Since this position is decisive for the adjustment ofampere-tums of the condenser II, the latter condenser must also be givena new adjustment. In apparatus according to the invention such doubleadjustment is avoided by leaving the constant magnetomotive force of thecondenser l unchanged when varying the illumination aperture of theobject by exchanging the pole shoe system located in the operativeposition for a pole shoe system of different imaging length, the variouspole shoe systems being matched to prevent shifting the axial positionof the first image r! of the beam source.

It is to be taken into account, however, that to some slight extent adisplacement of the plane of the first image rl may occur depending uponwhether the pole shoe system has its main planes virtually coincidentwith the lens center, i.e. whether a so-called "thin" lens is involved,or whether the pole shoe system inserted into active position of thecondenser I constitutes a thick lens so that the axial extent of thelens field can no longer be neglected relative to the distance from thenext following condenser lens II. Some other properties in this respectare encountered with pole shoe systems whose bore diameters and gapwidths coincide as to order of magnitude with the corresponding data ofthe objective lens. For explaining these properties, also investigatedin the abovementioned paper of Liebmann and, Grad, reference will bemade to FIG. 2.

The two pole shoes I and 2 of the condenser I shown in FIG. 2 produce alens field of an approximately bell-shaped cross section, for example.The condenser II, not shown in this illustration, sees the virtual imagerl of the beam source. The actual intersection 3 of the corpuscularbeam, for example an electron beam e, with the lens axis a is shiftedbackwards to the point 3' due to the effect of the tail-field of thecondenser I, this tail-field being identified by crosshatching. Thiseffect occurs only with a condenser lens whose pole shoe system has sucha dimensioning that there occurs tail-field which causes curving of thebeam e.

In such a condenser I, whose lens field has an appreciable axial extentin comparison with the distance from the next condenser lens, as well aswith a condenser I whose main planes virtually coincide with the lenscenter (thin lens with k in the order 0H). l the differences in theaxial position of the virtual first pictures of the beam source producedwith respectively different pole shoe systems. are preferablycompensated by corresponding differences in the axial position of thelens gap in the different pole shoe systems. That is, the gaps of thedifferent pole shoe systems, mounted on a holder structure that extendstransverse to the corpuscular beam, are arranged at respectivelydifferent heights for the different systems. assuming that the beam axisis vertical and the plane of the pole shoe holder is horizontal.

If all of the pole shoe systems of the first condenser lens have adiameter D (FIG. 2) of their pole shoe bore and have a lens gap width Sin the order of magnitude of the corresponding diameter and width valuesof the objective lens, such a mutual displacement or staggering of thedifferent pole shoe systems in the axial direction can be dispensed withby selecting the excitation of the condenser I in such a manner thatthis condenser has at least approximately the minimal focal length. Thiswill be understood from the following. As can be seen from the curvesknown from the paper by Liebmann and Grad (Page 963), small changes ofthe parameter k do not result in appreciable displacements of thehere-interesting back focal point (on the image-forming side) of anelectromagnetic lens, this focal point being located approximately inthe lens center when the lens is operated in the region of its minimalfocal length, i.e. where the curve of the focal length exhibits anextreme value.

For a given beam voltage, the lens current required for obtaining theminimum focal length varies only little for different ratios of gapwidth S to bore diameter D. If one wants to exactly maintain anoperation of the condenser I at minimal i focal length with any one ofits pole shoe systems, without 'necessity forchanging the lens current,then care should be taken that all of the pole shoe systems possess thesame ratio of gap width S to bore diameter D.

To avoid or minimize lens faults, it has been found advisable to givethe gap width and the bore diameter of the pole shoe system in condenserII the same dimensions, at least as to the order of magnitude, and thesedimensions should be preferably in the same order of magnitude as thefocal length. In practice, a value of mm. has been found suitable.

The area limit diaphragm is preferably arranged at a relatively largedistance from the entrance pupil plane of the objective lens. That is,when using a separate condenser lens III, the diaphragm should besituated far ahead of the front focal plane. As a rule, this distanceamounts to more than times the focal length of the condenser III. Adistance in the order of magnitude of 200 mm. has been found suitable.

For accommodating the plurality of different pole shoe systems, theassembly of condenser I is preferably equipped with a holder structurewhich carries the pole shoe systems and is displaceable in a planetransverse to the axis of the lens system. Preferably the holderstructure is designed as a turntable rotatable about an axis spaced fromand parallel to the axis of the corpuscular beam. Rotation of the tablethen serves to place the individual pole shoe systems into or out of theoperative position. In principle such holders and turntables are knownas such for projective lenses.

Since the condenser I, particularly in a corpuscular beam apparatusaccording to the principle shown in FIG. I, may be located at relativelygreat height, it is advisable to effect the exchange of the pole shoesystems with the aid of actuating motors which can be controlled by theattending person, for example when sitting in front of the microscopeapparatus. If desired, however, a mechanical transmission with the aidof control rods or other links leading from an actuator or handle in thevicinity of the attending person to the condenser I may also beprovided, or the driving members may be arranged directly in thevicinity of the condenser I.

The holder structure or turntable may be equipped with electric contactswhich effect an indication of the fact that a pole shoe system has beenplaced in accurate operating position, or which cause the actuatingmotors to stop upon termination of the pole shoe exchange.

Preferably the entire assembly of condenser l. including the holder orturntable for the pole shoe systems. is adjustably mounted so as to bedisplaceablc in directions transverse to the axis of the corpuscularbeam.

The features just described are embodied in the condenser assemblyaccording to the invention illustrated in FIGS. 3 and 4. This assemblyis applicable in electron microscopes, for example in a microscope ascompletely illustrated in the abovementioned publication of the SixthInternational Congress for Electron Microscopy. I966.

The condenser 1 comprises an excitation winding surrounded by a magneticiron circuit for conducting the magnetic flux of the lens, this circuitbeing composed of parts 21 to 26. The flux passes through the insertedpole shoe system 27, containing two pole shoes 28 and 29. The two poleshoes are joined together by a nonmagnetic sleeve 30, for example ofbrass, so as to form a rigid structural unit. The pole shoes havecoaxially aligned central bores 31 and 32 respectively and form betweeneach other a lens gap 33 in which the magnetic field of the condenserlens 1 acts upon the electron beam whose axis is denoted by 34.

The lens is equipped with four pole shoe systems of which in FIG. 3 onlythe systems denoted by 27 and 35 are visible. The pole shoe systems areinserted into a turntable generally denoted by 36. The table isrotatable about an axle member 37 by means of a driving gear 38 so thatselectively one of the pole shoe systems is turned into the beam path.The axle member 37 is journaled at the top in a plate 39 of magneticallyineffective material such as brass. The bottom end of the axle member 37is joumaled in another plate 40 likewise of magnetically ineffectivematerial.

The rotary motion of the driving wheel 38 is transmitted to theturntable 36 through a drive shaft 41, passing through a sleeve 42 andvacuum-tightly sealed by means of a sealing gasket 43. The shaft carriesa spur gear meshing with an annular gear or rack 45 fastened to theturntable. Spring catches are preferably provided for marking orsecuring the proper operating positions of the individual pole shoesystems, these catches not being visible in the illustrations.

The entire lens assembly, inclusive of the turntable 36 with the poleshoe systems and the appertaining drive means, is mounted in the column46 that forms part of the evacuated housing of the electron microscopeand is displaceable transversely to the beam axis 34. As will be seenfrom the arrange ment of seals 47, 48, 43, 49, 50, and 51, the topportion of the lens assembly is largely located outside of the vacuumspace of the electron microscope, whereas a ring-shaped chamber 54 islocated beneath the bottom of the lens assembly, namely between adisc-shaped member 52 and a fixed support 53 carrying the entire lensassembly. More specifically, the lens assembly rests with its bottommember 24 upon a ring 55 of hardened steel or the like which is placedupon an annular shoulder formed by the fixed member 53 of the column orhousing structure of the microscope.

The chamber 54 can be selectively connected, for example with the aid ofa three-way valve or cock, with the ambient air or with a vessel of lowpressure, such as with the prevacuum tank of the electron microscope.When the pressure in chamber 54 is increased by having it communicatewith the ambient air, the contact pressure between the lens assembly andthe ring plate 55 is reduced to facilitate a transverse displacement ofthe lens assembly. When thereafter the pressure in chamber 54 isdecreased by connecting it with the vacuum space, the contactingpressure between the lens assembly and the ring plate 55 is increased inorder to rigidly fix the lens assembly to the supporting structure. Theduct and valve connections for thus controlling the pressure in chamber54 are not illustrated. If desired, reference as to these details may behad to the copending application Ser. No. 656,557, filed July 27, 1967,of which i am a coinventor.

For reducing the contacting force between the lens assembly and itsfixed support, there is provided a sprung ball arrangement 56.Preferably three such ball and spring arrangements are mounted in arotationally symmetrical grouping with respect to the beam axis 34. Theeffects of the springs and of the controllable pressure in the chamberbecome superimposed upon each other in such a manner that the springarrangements 56 furnish a constant share of the contacting pres sureacting upon the ring 55, whereas the controllable pressure in chamber 54supplies a variable share of this contacting pressure. Relative to thiscoaction of the chamber 54 with the spring devices. reference may alsobe had to the copending applicationjust mentioned.

The beam source of the microscope is mounted on top of the portion 57located above the lens assembly proper and corresponding to theabove-mentioned member 53.

The means for effecting the transverse displacement of the pole piecesystems are best apparent from FIG. 4. The illustrated section is takenat the height of the actuating motors 58 and 59 which effect thetransverse displacement of the condenser lens assembly denoted by 60 inFIG. 4. The motors act through rollers 61 and 62 upon the iron member ofthe magnetic circuit in opposition to the force of restoring springs 63and 64. These springs are constituted by respective stacks of discsprings. Each stack is provided with a roller 65 or 66, thus permittingany displacements of the lens assembly 60 in a plane transverse to theaxis 34 of the electron beam.

The driving wheel or gear 38 for the turntable 36 may be replaced by anactuating motor. Furthermore, the actuating wheel proper or a controlknob may be arranged in the servicing desk for the electron microscopeto act upon the turntable through a mechanical or electricaltransmission which transfers the rotation of the wheel or knob to theturntable 36.

As explained, the four pole shoe systems, of which only ,those denotedby 27 and 35 are visible in FIG. 3, have different imaging lengthsrespectively but have the axial position of the lens gaps, the diameterof the pole shoe bores and the width of the lens gaps so dimensionedthat the axial position of the first image of the beam source remainsinvariable when exchanging one inserted pole shoe system for any one ofthe other three available systems.

The independent adjustment of illumination aperture of the object, onthe one hand, and illuminated object area, on the other hand, is alsoattained with the aid of the invention when employing a beam path of thetype indicated in the periodical OptilC' 1962, pages 273-286. Such abeam path is distinguished by the fact that the above-described arealimit diaphragm is substituted by the reduced image of the beam sourceformed by means of the condenser l and, as the case may be, by furthercondenser lenses, and that a diaphragm that determines the illumination(radiation) aperture of the object is arranged in the entrance pupilplane of the objective lens. In this case, the change of pole shoesystems in condenser I according to the invention has the result ofchanging the size of the illuminated region of the object.

To those skilled in the art it will be apparent from a study of thisdisclosure that my invention is amenable to a variety of modificationsand may be given embodiments than particularly illustrated and describedherein, without departing from the essential features of my inventionand within the scope of the claims annexed hereto.

1 claim:

1. Corpuscular beam apparatus, comprising in axial alignment acorpuscular beam source, condenser lens means with a first condenserlens facing the source for producing a reduced image thereof, and anobjective lens defining an object plane, said first condenser lenscomprising a fixed magnetic circuit structure and a plurality of lenspole shoe systems selectively placeable into active position relative tosaid fixed structure so as to be axially aligned with said source andsaid objective lens, each of said pole shoe systems having a centralbore for the passage of the electron beam and an interpole lens gap, andsaid pole shoe systems having respectively different image distances andhaving the diameter of said bore and the width and axial position ofsaid lens gap adapted to maintain the axial position of said sourceimage substantially invariable regardless of which of said pole shoesystems is placed into said active position.

2. Corpuscular beam apparatus according to claim I. comprising acorpuscular beam source. a first condenser lens producing a reducedimage of said source. an apertured diaphragm coaxially following saidfirst condenser lens in the beam direction, further condenser lens meanscomprising a last condenser lens to produce a reduced image of thediaphragm aperture on an object plane. an objective lens defining saidobject plane. said first condenser lens having a plurality of lens poleshoe systems selectively placeable into active position of coaxialalignment with said last condenser lens and objective lens, said poleshoe systems having respectively different image distances and havingeach a central pole shoe bore and an interpole lens gap, and said poleshoe systems having the diameter of said bore and the width and axialposition of said lens gap adapted to each other so as to maintain theaxial position of said source image substantially invariable regardlessof which one of said pole shoe systems is placed into said activeposition.

3. In apparatus according to claim 2, said first condenser having itsmain electron-optical planes substantially coincide with the lenscenter; and said pole shoe systems of said first condenser lensdiffering from each other substantially in the axial position of saidrespective lens gaps to thereby compensate for differences in the axialposition of said source image.

4. In apparatus according to claim 2, said first condenser having anaxially extended lens field as compared with its distance from saidfurther condenser lens means; and said pole shoe systems of said firstcondenser lens differing from each other substantially in the axialpositions of said respective lens gaps to thereby compensate fordifferences in the axial position of said source image.

5. In apparatus according to claim 2, said bore diameter and lends gapwidth in at least two of said pole shoe systems being in the same orderof magnitude as the bore diameter and gap width respectively of saidobjective lens, and said first condenser having when in operation anexcitation for at least approximately the minimal focal length.

6. In apparatus according to claim 1, all of said pole shoe systems ofsaid first condenser lens having the same ratio of lens gap width tobore diameter.

7. In apparatus according to claim 2, said last condenser lens forming alens assembly distinct from that of said objective lens and having afocal plane substantially coincident with the entrance pupil plane ofsaid objective lens.

8. In apparatus according to claim 2 said objective lens having prefieldwhich forms said last condenser lens.

9. In apparatus according to claim 2. said further condenser lens meanscomprising another condenser lens located between said first and saidlast condenser lenses and of longer focal length than said lattercondenser lenses so as to transfer said source image at leastapproximately to the entrance pupil plane of said objective lens.

10. In apparatus according to claim 9, said other condenser lens havinga lens gap width and a lens bore diameter whose respective sizes are ofthe same order of magnitude.

11. In apparatus according to claim 10. said other condenser lens havinga lens gap width and a lens bore diameter of the same order of magnitudeas its focal length.

12. In apparatus according to claim 2, said diaphragm being spaced fromthe entrance pupil plane of said objective lens a distance in the orderof magnitude of I00 times the focal length of said last condenser lens.

13. Apparatus according to claim 1, comprising a holder structuredisplaceable relative to said lenses in a plane perpendicular to thelens axis, said pole shoe systems being mounted on said holder structurefor selectively placing said systems to the active position bydisplacing said holder structure.

14. In apparatus according to claim 13, said holder structure comprisinga turntable having an axis of rotation spaced from said lens axis andparallel thereto, whereby rotational displacement of said turntableplaces said pole shoe systems into and out of said active position.

15. Apparatus according to claim 14, comprising an electric drivecoupled with said turntable for rotatingthe latter.

16. In apparatus according to claim 13, said first condenser lenscomprising a magnetic circuit assembly, said holder structure beingdisplaceably mounted on said assembly to form a structural unit togethertherewith, and said unit being displaceable as a whole for adjustmenttransversely of the beam axis.

17. Apparatus according to claim 1, comprising a radiation aperturediaphragm in the entrance pupil plane of said objective lens fordetermining the irradiation aperture of the object being imaged.

1. Corpuscular beam apparatus, comprising in axial alignment a corpuscular beam source, condenser lens means with a first condenser lens facing the source for producing a reduced image thereof, and an objective lens defining an object plane, said first condenser lens comprising a fixed magnetic circuit structure and a plurality of lens pole shoe systems selectively placeable into active position relative to said fixed structure so as to be axially aligned with said source and said objective lens, each of said pole shoe systems having a central bore for the passage of the electron beam and an interpole lens gap, and said pole shoe systems having respectively different image distances and having the diameter of said bore and the width and axial position of said lens gap adapted to maintain the axial position of said source imaGe substantially invariable regardless of which of said pole shoe systems is placed into said active position.
 2. Corpuscular beam apparatus according to claim 1, comprising a corpuscular beam source, a first condenser lens producing a reduced image of said source, an apertured diaphragm coaxially following said first condenser lens in the beam direction, further condenser lens means comprising a last condenser lens to produce a reduced image of the diaphragm aperture on an object plane, an objective lens defining said object plane, said first condenser lens having a plurality of lens pole shoe systems selectively placeable into active position of coaxial alignment with said last condenser lens and objective lens, said pole shoe systems having respectively different image distances and having each a central pole shoe bore and an interpole lens gap, and said pole shoe systems having the diameter of said bore and the width and axial position of said lens gap adapted to each other so as to maintain the axial position of said source image substantially invariable regardless of which one of said pole shoe systems is placed into said active position.
 3. In apparatus according to claim 2, said first condenser having its main electron-optical planes substantially coincide with the lens center; and said pole shoe systems of said first condenser lens differing from each other substantially in the axial position of said respective lens gaps to thereby compensate for differences in the axial position of said source image.
 4. In apparatus according to claim 2, said first condenser having an axially extended lens field as compared with its distance from said further condenser lens means; and said pole shoe systems of said first condenser lens differing from each other substantially in the axial positions of said respective lens gaps to thereby compensate for differences in the axial position of said source image.
 5. In apparatus according to claim 2, said bore diameter and lends gap width in at least two of said pole shoe systems being in the same order of magnitude as the bore diameter and gap width respectively of said objective lens, and said first condenser having when in operation an excitation for at least approximately the minimal focal length.
 6. In apparatus according to claim 1, all of said pole shoe systems of said first condenser lens having the same ratio of lens gap width to bore diameter.
 7. In apparatus according to claim 2, said last condenser lens forming a lens assembly distinct from that of said objective lens and having a focal plane substantially coincident with the entrance pupil plane of said objective lens.
 8. In apparatus according to claim 2, said objective lens having prefield which forms said last condenser lens.
 9. In apparatus according to claim 2, said further condenser lens means comprising another condenser lens located between said first and said last condenser lenses and of longer focal length than said latter condenser lenses so as to transfer said source image at least approximately to the entrance pupil plane of said objective lens.
 10. In apparatus according to claim 9, said other condenser lens having a lens gap width and a lens bore diameter whose respective sizes are of the same order of magnitude.
 11. In apparatus according to claim 10, said other condenser lens having a lens gap width and a lens bore diameter of the same order of magnitude as its focal length.
 12. In apparatus according to claim 2, said diaphragm being spaced from the entrance pupil plane of said objective lens a distance in the order of magnitude of 100 times the focal length of said last condenser lens.
 13. Apparatus according to claim 1, comprising a holder structure displaceable relative to said lenses in a plane perpendicular to the lens axis, said pole shoe systems being mounted on said holder structure for selectively placing said systems to the active position by displacing said holder structure.
 14. In apparatus according to claim 13, said holder structure comprising a turntable having an axis of rotation spaced from said lens axis and parallel thereto, whereby rotational displacement of said turntable places said pole shoe systems into and out of said active position.
 15. Apparatus according to claim 14, comprising an electric drive coupled with said turntable for rotating the latter.
 16. In apparatus according to claim 13, said first condenser lens comprising a magnetic circuit assembly, said holder structure being displaceably mounted on said assembly to form a structural unit together therewith, and said unit being displaceable as a whole for adjustment transversely of the beam axis.
 17. Apparatus according to claim 1, comprising a radiation aperture diaphragm in the entrance pupil plane of said objective lens for determining the irradiation aperture of the object being imaged. 